Rotational molding and casting techniques are quite highly developed and are well known in the molding and casting art. The rotational molding of the synthetic resins in powdered form is likewise well known. It is also known in the prior art to rotationally mold articles utilizing a mixture of thermoplastic powder and thermoplastic pellets, such as for instance a mixture of polyethylene pellets and powder, to provide rotationally molded, hollow articles such as containers, Christmas tree ornaments and other decorative objects other than playballs or sport balls. U.S. Pat. Nos. 3,293,344, 3,368,013, 3,474,165, 3,514,508, 3,564,656, 3,788,792 and 3,914,361 disclose various processes and/or apparatus for rotational molding of thermoplastic hollow articles.
However, to applicant's knowledge, no one heretofore has thought of utilizing a mixture of plastic pellets and powder, both of which are copolymers of ethylene and vinyl acetate and, having a predetermined range of powder to pellet ratio and a predetermined melt index, for producing a thermoplastic rotationally molded hollow article which possessed good to excellent "bounce" characteristics substantially independent of air pressure interiorly of the article.
The synthetic thermoplastic material most commonly used for producing hollow articles such as playballs by the rotational casting process is vinyl plastisol. Such vinyl playballs have relatively poor "bounce" characteristics unless they are internally pressurized, as for instance with air. Since most thermoplastics are at least slightly permeable to air, and such pressurized articles or balls may leak air pressure at their closure plugs or valves, such playballs made by such prior art methods gradually lose their internal air pressure, becoming progressively more flabby and shrinking, resulting in poor "bounce". The average useful life of such inflated thermoplastic playballs is not much longer than one year from the date of manufacture.
Likewise, hollow molded articles such as playballs or sport balls made from rubber-like compositions of natural and/or synthetic polymers (e.g. conventional tennis balls) depend on internal air pressure for their required "bounce" characteristics and rapidly become unfit for use unless equipped with a valve for re-inflation.
Another type of small playballs used by children is made from solid cellular thermoplastics or from rubber-like compositions of natural and/or synthetic polymers (e.g. sponge rubber). Such solid cellular balls possess relatively good "bounce" characteristics but are heavier than desired for indoor use by children and have unattractive and easily abraided surface textures.